1. Field of the Invention
The present invention relates to a passenger detection system, especially relates to a passenger detection system which may optionally perform a seat belt reminder function of reminding a passenger to fasten their seat belt, based on a seat-sitting situation of whether every passenger is sitting on their car seat.
Priority is claimed on Japanese Patent Application No. 2009-222723, filed Sep. 28, 2009, the contents of which are incorporated herein by reference.
2. Description of Related Art
A seat belt device is provided on a car seat, so that the passenger is fastened to their seat by a seat belt to hold the passenger in the seat even when the car rapidly slows down due to a collision. To avoid that a passenger is not wearing the seat belt, a seat belt non-wearing alarm device or a seat belt reminder system can be used. The seat belt reminder system notifies that a passenger is not wearing their seat belt. The notification may be given by either a visual alarm which acts on the passenger's visual sense by lighting a lamp, or an audio alarm which acts on the passenger's acoustic sense by loud buzzing. In Japan, on Sep. 1, 2005, the rules that a driver must obey were enforced. Under the rules, a car manufacturer must install a seat belt reminder system in their car. The seat belt reminder system will alarm and remind a driver or passenger to fasten their seat belt when the driver's seat belt is unfastened.
The seat belt reminder system generates an alarming sound to remind a passenger to fasten their seat belt. The generation of an alarming sound is made based on first and second types of information. The first type of information indicates that a passenger is sitting in their seat. The second type of information indicates that a buckle switch of the seat is OFF.
A detection system to perform passenger detection may be classified into first and second detection systems. The first detection system uses a weight sensor or a piezoelectric sensor both of which determine the weight of a passenger. The second detection system uses a capacity sensor which measures the body-size of a passenger or the surface area of a passenger.
FIG. 7A is a schematic diagram illustrating the principles of the passenger detection system which uses the detection system with the capacity sensor. In this detection system, a weak electric field (EF) is generated at an antenna electrode provided in the seat, such that electric charges are generated on the surfaces of the electrode and an object to be measured. A capacitive coupling is caused between the surface of the electrode and the surface of the object to be measured. Therefore, the detection system is able to measure variation of displacement current which depends on variation of a capacity. The capacity variation varies depending on the body-size of a passenger and the seat-sitting situation of the passenger.
As shown in FIG. 7A, a low voltage of a high frequency is generated by a sine wave oscillator OSC and applied to an antenna electrode E1, so that a weak electric field (EF) is generated around the antenna electrode E1, resulting in a displacement current I which flows on the side of the antenna electrode E1. The value of the displacement current I depends on the value of a static electric capacity which is caused between the antenna electrode E1 and an object OB. Therefore, the value of the displacement current I depends on the dielectric constant of the object OB to be measured. The object is placed near the antenna electrode E1. Therefore, the current which flows on the side of the antenna electrode E1 varies depending on weather the object OB to be measured is on the seat or not. By using this phenomenon, it is possible to detect the seat-sitting situation of whether a passenger is sitting in their seat or not. The detection is made based on a physical quantity such as displacement current obtained by the sensor.
As shown in FIG. 7B, a plurality of antenna electrodes are provided in the car. For example, the antenna electrode E2 is provided on the seating face of the seat, and the antenna electrodes E3 and E4 are provided on the back face of the seat. Therefore, it is possible to obtain many physical quantities about the object (passenger) to be measured on the seat, such that the seat-sitting situation of the passenger on the seat is appropriately detected.
For example, FIG. 1 of Japanese Patent No. 3779069 discloses a technique which a plurality of antenna electrodes are provided on the back face of the seat in predetermined period.
To use information obtained by the sensors as determination results which indicate whether the passenger is sitting on the seat or not, it is necessary to use a determination device which includes a control circuit. The control circuit regularly obtains data from the sensor, and compares a predetermined threshold with a physical quantity (a current value or a capacity measured value calculated from the current value) which is a measured result based on a weak electric field technique.
For example, FIG. 3 of Japanese Patent No. 3458323 discloses a technique of a passenger detection system. The passenger detection system lights an alarm lamp, based on the determination result which indicates whether the passenger is sitting on the seat or not and the determination result which indicates whether the passenger is wearing the seat belt or not.